Methods and systems for minimizing NOx and CO emissions in natural draft heaters

ABSTRACT

Systems and methods for reducing NOx and CO emissions in a natural draft heater are disclosed. For example, the disclosure provides embodiments of systems and methods for controlling a draft value within a heater shell to deliver an amount of excess air to a burner to thereby maintain at least one of NOx emissions not exceeding 0.025 lb/MMBtu (HHV) and CO emissions not exceeding 0.01 lb/MMBtu (HHV) in a natural draft heater.

FIELD OF THE DISCLOSURE

The disclosure herein relates to systems and methods for minimizing NO_(x) and controlling CO emissions in natural draft heaters.

BACKGROUND

Fired heaters are well-known and extensively used in the oil and gas industry. Generally, fired heaters are direct-fired heat exchangers that use hot combustion gases to raise the temperature of a process fluid flowing through heating coils arranged inside the heater. Fired heaters typically contain one or more burner air registers therein, which control the mixing of fuel and air during the fuel combustion process. The fired heaters may be designed to use refinery fuel gas and/or city natural gas and may operate under a wide range of operating conditions.

Fired heaters are used in several industrial processes and are designed in various configurations to meet unique applications associated with a given industrial process. Irrespective of design, fired heaters commonly contain at least three main components, including a heating coil, an enclosed structure (e.g., including a firebox and a stack), and combustion equipment (e.g., including one or more burners/air registers). The heating coil may include tubes connected together in series that carry the charge (e.g., process fluid) that is being heated, for example, when heat within the firebox is transferred to the charge passing through the tubes. The heating coil absorbs the heat mostly by radiant heat transfer and convective heat transfer from gases (e.g., flue gases), which are vented to the atmosphere through the stack. The firebox is commonly a structure (e.g., constructed of steel) that forms an enclosure lined with a refractory material that holds the heat that is generated by burning a fuel in one or more burners. The one or more burners may be positioned either in the floor of the firebox (e.g., in a vertical draft heater) or on the sidewalls of the firebox (e.g., in a horizontal draft heater). Combustion air is typically drawn through the system from the outside atmosphere and various control instruments and schemes are commonly provided to control the fuel firing rate and air flow through the system to maintain the desired operating conditions.

Burning or combustion of the fuel gas in a fired heater generally results in an exothermic reaction from the rapid combination of oxygen with fuel (oil or gas). Most fuels used in fired heaters contain hydrocarbons and at least some amount of sulfur. Since perfect mixing of stoichiometric quantities of fuel and air is not feasible, excess air is needed to ensure complete fuel combustion in the fired heater. Excess air is commonly expressed as a percentage of theoretical quantity of air required for perfect stoichiometric combustion. It is generally undesirable to operate with less than stoichiometric combustion air, as such operation may lead to a smoking stack and will cause incomplete combustion of the fuel. Incomplete combustion does not provide the maximum amount of energy from the fuel. Further, when fuel is combusted with insufficient air, undesirable components such as carbon monoxide (CO) and hydrogen will appear in the flue gases.

The configuration and/or design of fired heaters may vary and any given fired heater is commonly classified with respect to its draft design. API Standard 560 defines “draft” as the negative pressure or vacuum of the air and/or flue gas at any point in the heater. Generally, hot flue gases within the firebox and stack are lighter than the relatively cold ambient air, thus resulting in a slightly negative pressure inside the heater. Common fired heater draft designs include, but are not limited to, forced draft heaters, induced draft heaters, balanced draft heaters, and natural draft heaters. In a forced draft heater, air is supplied using a centrifugal fan commonly referred to as a forced draft (FD) fan, which forces air through the system. Such heater configurations generally provide for high air velocity, better air/fuel mixing, and small overall burner size. However, the stack is still needed, in addition to the fan, to create a negative draft inside the furnace. In an induced draft heater, an induced draft (ID) fan is typically used to remove flue gas from the heater and to pull air through the burner and into the combustion zone. A negative pressure inside the furnace ensures air supply to the burners from the atmosphere. In a balanced draft heater, both an FD fan and an ID fan are used within the heater to push and pull combustion air through the burner and into the combustion zone. In this way, the amount of air delivered by the FD fan and the ID fan may be controlled and/or balanced. Natural draft heaters are the most common type of fired heaters. In a natural draft heater, air is drawn into the heater by a draft created by the stack. Generally, the stack height controls the draft created within the natural draft heater (e.g., the higher the stack, the higher the draft created therein). The negative pressure differential created by the draft generally allows for combustion air to be drawn into the burners, through the firebox, with the flue gases eventually flowing out of the stack.

It is important to minimize emissions and, in particular, NO_(x) and CO emissions, from fired heaters, as these emissions limits are heavily regulated by governmental agencies, such as the Environmental Protection Agency (EPA) in the United States. For example, recent environmental regulations have created a requirement for reduction of NO_(x) and CO emissions for natural draft heaters that has not been previously achievable in industry. Reduction of NO_(x) and CO emissions presents various process challenges (especially in existing heaters) and may be extremely costly. Those in the industry are continuously looking for cost-efficient ways to reduce these emissions both in existing and new fired heaters to comply with constantly changing regulations.

SUMMARY OF THE DISCLOSURE

The disclosure herein provides one or more embodiments of systems and methods for reducing NO_(x) and CO emissions in fired heaters through incorporation of various design specific components/equipment and/or operational schemes. In particular, the disclosure provides design modifications for new and/or existing fired heaters to reduce NO_(x) and CO emissions therefrom, which may be used independently or in various combinations. Such systems and methods, when used in combination, may advantageously provide NO_(x) emissions from natural draft heaters not exceeding 0.025 lb/MMBtu higher heating value (HHV) and CO emissions from natural draft heaters not exceeding 0.01 lb/MMBtu higher heating value (HHV).

In one or more aspects, the disclosure provides one or more natural draft heater systems. An embodiment of a system, for example, may include a heater shell having a base and designed to circulate a flue gas internally therein via negative pressure. In some embodiments, the flue gas may be generated by combustion of a fuel within the heater shell. Another embodiment of a system, for example, may include one or more heating coils positioned within the heater shell. In some embodiments, the one or more heating coils may contain a process fluid therein and the one or more heating coils may be arranged to transfer heat from the circulated flue gas to heat the process fluid. Some embodiments of systems may include a draft sensor positioned within the heater shell to measure a negative pressure of the flue gas within the heater shell during operation of the natural draft heater.

An embodiment of a system, for example, may include a stack attached to the heater shell for venting of at least a portion of the circulated flue gas to atmosphere. In some embodiments, the stack may include an outer shell and a split-range stack damper positioned within the outer shell to maintain a negative pressure of the flue gas being vented from the natural draft heater. In still other embodiments, the split-range stack damper may have one or more components, for example, a set of inner blades, a set of outer blades, and one or more actuators arranged to actuate the set of inner blades and the set of outer blades to thereby effectuate movement thereof.

Some embodiments of systems as described herein may include, for example, at least one burner assembly connected proximate the base of and within the heater shell to combust the fuel when supplied thereto. In some embodiments, combustion of the fuel in the at least one burner assembly may generate flue gas that transfers heat to the process fluid contained within the one or more heating coils. Generally, the at least one burner assembly in the systems of the disclosure include one or more different components. In some embodiments, for example, the at least one burner assembly may include a burner positioned within the at least one burner assembly to ignite the fuel when being supplied to the burner. In some embodiments, the at least one burner assembly may include a burner air sensor positioned adjacent the burner to measure a level of excess air. In some embodiments, the at least one burner assembly may include an air plenum adjacent to the burner to distribute air into the burner, the air plenum including an air input to receive atmospheric air. In still other embodiments, the at least one burner assembly may include a burner air register in fluid communication with the air input to direct the atmospheric air into the air plenum. In certain embodiments, the burner air register may have a housing and one or more plates attached to the housing and positioned in fluid communication with the air plenum to direct air flow into the plenum. In some embodiments, the burner air register may have a handle attached to the housing thereof to adjust the position of the one or more plates to effectuate a movement thereof. In some embodiments, each of the one or more plates may be configurable between one or more of an open position, a partially open position, and a closed position to selectively supply air to the air plenum.

An embodiment of a system, for example, may also include a fuel supply system in the at least one burner assembly. In some embodiments, for example, the fuel supply system may include at least a fuel input conduit to deliver fuel to the burner. In some embodiments, for example, the fuel supply system may include a primary manifold assembly and optionally a second, staged manifold assembly. In certain embodiments, the fuel input conduit may be positioned in fluid communication with the primary manifold assembly. In some embodiments, the primary manifold assembly may be positioned in fluid communication with one or both of a burner tip of the burner and the second, staged manifold assembly to deliver fuel to one or both of the burner tip of the burner and the second, staged manifold assembly. In some embodiments, the second, staged manifold assembly may be positioned in fluid communication with another burner tip to deliver fuel to the another burner tip. In yet other embodiments, the second, staged manifold assembly may include a staged manifold valve configurable to be in a closed position to shut off fuel flow or in an at least partially open position to direct a preselected amount of fuel to the another burner tip to achieve a desired concentration of air and fuel mixture in the another burner tip.

An embodiment of a system, for example, may include a controller in electrical communication with the various components within the natural draft heater system. For example, in some embodiments, the controller may be in electrical communication with at least the burner air register, the burner air sensor, the draft sensor, and the one or more actuators in the split-range stack damper to control the negative pressure of the flue gas within the heater shell to deliver an amount of excess air to the burner to thereby maintain at least one of NO_(x) emissions not exceeding 0.025 lb/MMBtu (HHV) and CO emissions not exceeding 0.01 lb/MMBtu (HHV) in the natural draft heater.

Other embodiments of systems, for example, may include a controller capable of performing various functionalities. For example, in some embodiments, the controller may receive an input signal representative of negative pressure of the flue gas from the draft sensor. In some embodiments, the controller may then provide an output signal to the one or more actuators in the split-range stack damper to adjust the positioning of the set of inner blades and the set of outer blades of the split-range stack damper and thereby maintain the negative pressure of the flue gas to within a preselected range. For example, in some embodiments, the preselected range may be maintained in the range of about 0.10-inches water-column to about 0.15-inches water-column. In certain other embodiments, the controller may receive an input signal representative of excess air level from the burner air sensor. In some embodiments, the controller may provide an alert to an operator to adjust configuration of the one or more plates and thereby control the excess air in the burner to a value in the range of between about 15% to about 25% by weight based on the combined weight of air and fuel needed for complete combustion.

Further embodiments of systems, for example, may include a burner fuel tip positioned within the burner and a riser plate positioned adjacent thereto at least partially enclosing the burner fuel tip. In some embodiments, the riser plate may have a burner riser positioned proximate a base thereof and connected thereto by one or more riser welds to allow air to enter the riser plate between riser welds during operation of the natural draft heater. An embodiment of a system as described herein may also include, for example, one or more air rings positioned about the burner riser proximate the one or more riser welds to reduce air entering the riser plate. For example, in some embodiments, the one or more air rings are positioned to reduce air entering the riser plate when the natural draft heater is operating at rates greater than about 40% of design capacity. In certain other embodiments, natural draft heater systems as described herein may include a silicone seal applied at the connection between the burner risers and the riser plates to further prevent air infiltration into the burner thereby reducing NO_(x) formation in the heater shell.

An embodiment of a system, for example, may include a flame scanner positioned adjacent the air plenum to detect the presence of a flame in the burner. In some embodiments, a purge air injection input may be positioned adjacent the flame scanner to deliver purge air directly to the burner. For example, in such embodiments the purge air being may be used as combustion air by the burner thereby reducing NO_(x) formation within the heater shell.

Other aspects of the disclosure provide methods of reducing at least one of NO_(x) and CO emissions in natural draft heaters. An embodiment of a method, for example, includes measuring a selected draft value related to the negative pressure of flue gas within the heater shell when operation of the natural draft heater occurs. Another embodiment of a method may include adjusting a split-range stack damper associated with the natural draft heater to maintain the selected draft value to within a preselected range. For example, in some embodiments, the preselected range of the selected draft value may be maintained in the range of about 0.10-inches water-column to about 0.15-inches water-column to provide the desired negative pressure within the natural draft heater. Still another embodiment of a method may include measuring an excess air level in a burner when operation of the natural draft heater occurs. An embodiment of a method, for example, may include controlling excess air in a burner associated with the natural draft heater to a value in the range of between about 15% to about 25% by weight based on the combined weight of air and fuel needed for complete combustion within the burner. Some embodiments of a method, for example, may include controlling the draft within the heater shell within a preselected range thereby to control the amount of excess air in the burner and maintain at least one of NO_(x) emissions not exceeding 0.025 lb/MMBtu (HHV) and CO emissions not exceeding 0.01 lb/MMBtu (HHV) in the natural draft heater.

An embodiment of a method, for example, may include detecting the presence of a flame in the burner when operation of the natural draft heater occurs. Another embodiment of a method may include, for example, injecting a purge air directly into the burner, the purge air being used as combustion air by the burner to thereby reduce NO_(x) formation within the heater shell. Further embodiments of methods, for example, may include sealing one or more connections proximate the burner to prevent air infiltration into the burner to thereby reduce NO_(x) formation in the heater shell.

These and other features, aspects, and advantages of the disclosure will be apparent from a reading of the following detailed description together with the accompanying drawings, which are briefly described below. The disclosure includes any combination of two, three, four, or more features or elements set forth in this disclosure or recited in any one or more of the claims, regardless of whether such features or elements are expressly combined or otherwise recited in a specific embodiment description or claim herein. This disclosure is intended to be read holistically such that any separable features or elements of the disclosure, in any of its aspects and embodiments, should be viewed as intended to be combinable, unless the context of the disclosure clearly dictates otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the disclosure in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:

FIG. 1 is a schematic diagram of a natural draft heater according to an embodiment of the disclosure;

FIG. 2 is a perspective view of a large scale natural draft heater, including a plurality of burner assemblies positioned proximate a base of the heater shell, according to an embodiment of the disclosure;

FIG. 3A is a side elevation view of a burner assembly for use in a natural draft heater according to an embodiment of the disclosure;

FIG. 3B is an enlarged sectional view of two different components of the burner assembly as shown in FIG. 3A, including a pilot burner configuration and an air ring installed at the base of a burner riser, according to an embodiment of the disclosure;

FIG. 4A is a front elevation view of a burner assembly for use in a natural draft heater according to an embodiment of the disclosure;

FIG. 4B is an enlarged sectional view of a component of the burner assembly as shown in FIG. 4A, including a flame scanner positioned adjacent the air plenum, according to an embodiment of the disclosure;

FIG. 5A is an elevation view of an upper portion of a burner assembly, including a burner riser, a riser plate, a riser weld, and an air ring, according to an example embodiment of the disclosure;

FIG. 5B is an exploded perspective view of an air ring for installation in a burner assembly according to an embodiment of the disclosure;

FIG. 6 is a perspective view of a lower portion of a burner assembly, including an air plenum, a burner air register, a staged manifold assembly, a flame scanner, and a flame scanner, according to an embodiment of the disclosure;

FIG. 7 is a top plan schematic view of a split-range stack damper in electrical communication with a controller, according to an embodiment of the disclosure;

FIG. 8 is a data table that illustrates the calculated NO_(x) and CO emissions guarantees for a natural draft heater system according to an embodiment of the disclosure;

FIG. 9 is a graph of burner heat release versus measured NO_(x) showing the impact of various burner design parameters on NO_(x) missions in a natural draft heater system according to an embodiment of the disclosure; and

FIG. 10 is a graph of burner heat release versus measured NO_(x) showing the impact of various burner design parameters on NO_(x) emissions in a natural draft heater system according to an embodiment of the disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

The disclosure now will be described more fully hereinafter with reference to specific embodiments and particularly to the various drawings provided herewith. Indeed, the disclosure may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. As used in the specification, and in the appended claims, the singular forms “a,” “an,” “the,” include plural referents unless the context clearly dictates otherwise.

The disclosure provides embodiments of methods and systems for reducing NO_(x) and CO emissions in a fired heater (e.g., a natural draft heater). In particular, as will be provided in further detail below, the methods and systems relate to a combination of design enhancements and operational schemes that may, in some embodiments, be used in unison to lower NO_(x) and CO emissions in natural draft furnaces. This combination of design enhancements and operational schemes is potentially available for use in all new and/or existing fired heaters and, in particular, for use in all new and/or existing natural draft heaters to provide reductions in NO_(x) and/or CO emissions. The noted design enhancements and/or operational schemes may provide for substantial reductions in both NO_(x) and CO emissions in both existing and newly fabricated heaters, in some embodiments, to levels not previously achievable. In particular, certain combinations of operation and design attributes may result in a fired heater that operates with NO_(x) emissions not exceeding 0.0251 lb/MMBTU (HHV) and/or CO emissions not exceeding 0.01 lb/MMBTU (HHV). The noted design enhancements and/or operational schemes may also be effective to reduce at least one of NO_(x) and CO emissions in natural draft heater systems having a wide variety of operating conditions. In some embodiments, for example, such operation may range from heat absorption duty (100%) to turn down duty (20% of design duty). In some embodiments, the excess air level (the amount of air exceeding that required for complete combustion of the fuel) may be advantageously maintained between 15-25% for the entire operating range to maintain at least one of NO_(x) emissions not exceeding 0.025 lb/MMBTU (HHV) and/or CO emissions not exceeding 0.01 lb/MMBTU (HHV).

Generally, the methods and systems provided herein for reducing NO_(x) and CO emissions in heaters may be suitable for use on any type of natural draft heater commonly used in the industry. “Natural draft heater” as used herein, generally refers to any fired heater that uses flue gas buoyancy (e.g., a slightly negative pressure generated inside the heater which pulls atmospheric air therethrough) to support combustion of a fuel source therein. Natural draft heaters may be cylindrical or box type, vertically or horizontally configured, having a variety of different burner configurations and assemblies (e.g., with burners positioned on the sidewall or the floor of the heater), and may vary in height, dimensions, and/or material construction. In some embodiments, natural draft heaters may be designed to use refinery fuel gas and/or city natural gas as the combustible fuel source and may operate under a wide range of operating conditions. Typically, natural draft heaters include four sections: a burner/combustion section, a radiant section, a convective section, and a stack section.

FIG. 1 depicts a process diagram of a non-limiting, natural draft heater system 100 according to one or more embodiments of the disclosure. As shown in FIG. 1, the natural draft heater system 100 includes a heater shell 102 designed to circulate flue gas internally therein via negative pressure. In some embodiments, the flue gas being generated in the heater shell 102 is generated by combustion of a fuel, e.g., refinery oil/gas and/or natural gas. In one or more embodiments, such as the embodiment depicted in FIG. 1, the heater shell may include a base 106, a burner section 104, a radiant section 108, and a convective section 110. In some embodiments, the base 106 of the heater shell may sometimes be referred to as the floor of the heater shell, for example, when the heater shell in a vertical configuration such that the base of the heater shell is positioned at the bottom thereof. Generally, the terms “base” and/or “floor” and/or “bottom” are meant to be interchangeable as used herein in reference to a point in the heater shell. In some embodiments, the burner section 104 may be positioned proximate the base 106 of the heater shell 102 where combustion of the fuel occurs, for example. In some embodiments, the radiant section 108 may be positioned proximate the burner section 104, for example, to receive heat energy from the burner section and the radiate heat energy therefrom. In some embodiments, the convective section 110 may be positioned proximate the radiant section 108, for example, to provide convection from the radiant section.

In one or more embodiments, natural draft heater systems as described herein may include one or more heating coils 114 positioned within the heater shell. In some embodiments, the one or more heating coils 114 may be positioned within one or both of the radiant section 108 of the heater shell and the convective section 110 of the heater shell. For example, as depicted in FIG. 1, the one or more heating coils may be positioned in both the radiant section and the convective section. In some embodiments, one or more heating coils may be separately positioned in both of the radiant section and the convective section of the heater shell, or in other embodiments, a singular heating coil may extend into both the radiant section and the convective section. The configuration and/or arrangement of heating coils within the heater shell is not meant to be limiting and various heating coil configurations and/or arrangements are suitable for use in the heater shell as would be understood by those skilled in the art. In some embodiments, the one or more heating coils may contain a liquid process fluid therein (e.g., typically a fluid having a high heat transfer coefficient) that is capable of flow through the heating coils within the heater shell. Relevant process fluids are known in the industry and any such fluid may be employed in the systems and methods provided herein as will be understood by a skilled person in the art. Generally, when the heater is in operation, heated flue gases generated from combustion of the fuel in the burner travels upward through the radiant and convective sections of the heater, transferring heat to the liquid process fluid (e.g., via radiation and/or convection) in the heating coils, thereby retaining the transferred thermal energy from the flue gas in the process fluid such that it may be used elsewhere in the plant.

In one or more embodiments, natural draft heater systems as described herein may include a bridge wall 112 connected and/or attached to the heater shell 102 at one or more locations within the heater shell. In the embodiment depicted in FIG. 1, for example, the bridge wall 112 may be connected to the heater shell and positioned between the radiant section 108 and the convective section 110 thereof. Various monitoring equipment may be positioned proximate to the bridge wall in order to provide various measurements and/or emissions reporting functions. For example, an amount of excess oxygen (O₂) in the system, a concentration of combustible gas, flue gas temperature, and/or draft pressure within the system may be measured from the bridge wall 112. In some embodiments, natural draft heaters may be equipped with continuous emissions monitoring systems (CEMS) near the bridge wall which may measure, e.g., excess O₂, NO_(x), and CO emissions (e.g., for regulatory purposes). As noted above, natural draft heaters generally operate under negative pressure to pull ambient air through the heater shell during operation. In some embodiments, a draft sensor 116 may be positioned within the heater shell 102 to measure a negative pressure of the flue gas within the heater shell during operation of the natural draft heater. Typically, the draft sensor may be in the form of a pressure sensor positioned within the heater shell. In some embodiments, the draft sensor 116 may be positioned proximate the bridge wall 112, in particular, to measure a negative pressure at the bridge wall during operation of the natural draft heater, referred to herein as the “bridge wall draft” and/or a “bridge wall draft value,” for example. Generally, the location of highest draft pressure in a natural draft heater is typically at the top of the radiant section just below the first shield in the convection section, e.g., proximate to the bridge wall 112. Thus, if the pressure at the bridge wall (i.e., referred to herein as the “bridge wall draft value”) is slightly negative, the entire heater will be operating with the negative pressure. In some embodiments, the draft sensor may be in electrical communication with at least a control component 130 and one or more other components within the at least one burner assembly (e.g., a split-range stack damper 124) as will be discussed further herein.

In some embodiments, natural draft heater systems as described herein may include a stack 118 positioned proximate the convective section 110 of the heater shell 102 for venting of at least a portion of the circulated flue gas to the atmosphere. In some embodiments, the height and/or diameter of the stack may vary based on the desired draft in the heater shell and based on one or more operating conditions of the natural draft heater. Generally, the dimensions of the stack may vary as will be understood by a person of skill in the art. Referring back to FIG. 1, in one or more embodiments, the stack 118 may include an outer shell 122 and a split-range stack damper 124 positioned within the outer shell to maintain the bridge wall draft of the flue gas being vented from the natural draft heater. In some embodiments, the split-range damper may have a set of inner blades, a set of outer blades, and one or more actuators arranged to actuate the set of inner blades and the set of outer blades to thereby effectuate movement thereof, as will be discussed further herein with reference to FIG. 7. Advantageously, use of a split-range stack damper may provide for draft control within the natural draft heater across a wide range of operating conditions, as will be discussed further herein.

Common stack dampers have multiple blades that are connected through linkages and thus all blades move when a connected shaft is moved within the stack damper. This common type of stack damper configuration has a limited control range, that may not be as effective in fired heaters that require damper control across a wide range of conditions. However, the split-range damper used according to the systems and methods provided herein advantageously provides for a much higher level of draft control across a wider range of conditions than would be associated with common stack dampers, such as would be understood by a person of skill in this industry. FIG. 7 illustrates a detailed schematic, top-facing view of a split-range stack damper 300 as described herein. As depicted in FIG. 7, for example, the split-range damper 300 has four blades, a set of inner blades 302 operating at low firing rates, and a set of outer blades 304. In some embodiments, the set of inner blades 302 may be configured to operate at low firing rates (e.g., heat absorption duty less than about 50%, less than about 40%, less than about 30%, less than about 20%, or less) and the set of outer blades 304 typically remain closed during operation at low firing rates. However, when higher firing rates are utilized within the heater (e.g., heat absorption duty greater than about 40%, greater than about 50%, greater than about 60%, or more), the two outer blades 304 may also open to allow for more air flow within the system. In some embodiments, the set of inner blades and the set of outer blades may independently be configured to be closed and/or open and/or partially open (e.g., at varying degrees, such as from about 1% to about 99% open) to provide the desired draft conditions within the natural draft heater. For example, when the heater is started up (e.g., combustion occurs in one or more of the burner assemblies), the split-range damper is placed in full, open position (e.g., both the set of inner blades and the set of outer blades are completely open) providing for maximum draft and excess air to the burner. After start-up and as the firing range increases, the draft level within the heater may be tuned and controlled by adjusting the positioning of one or two sets of blades in the split-range damper.

In some embodiments, the split-range damper may also include a damper actuator 306 (or multiple actuators coupled to the individual blades) which are configured to adjust the positioning of the blades in response to control input from the controller 130. For example, the damper actuator 306 may be configured to output a current to the damper blades, based on control input from an operator, wherein each damper blade receives that current output and is configured to open and/or close to a certain degree based on the current output received. In some embodiments, for example, the outer blades 304 are designed to be fully open at a current of about 4 mA (milliamps) and fully closed at about 12 mA and the inner blades 302 are designed to be fully opened at a current of about 12 mA and fully closed at a current of about 20 mA. In such a configuration, as the pressure in the radiant section decreases during turn down, the outer blades start to close at a current of 4 mA while the inner blades remain fully open at a current of 4 mA. As the pressure further decreases within the radiant section, the inner blades will start to close when the current reaches 12 mA with the outer blades already being fully closed at a current of 12 mA. At start up, as the pressure increases within the radiant section, the inner blades will start to open at a current of 20 mA with the outer blades remaining fully closed at a current of 20 mA. At full operation, when the pressure within the radiant section is the highest, the outer blades will start to open at a current of 12 mA with the inner blades already being fully open at a current of 12 mA. Various other control designs and/or schemes may also be suitable for use in the split-range damper and generally, it should be noted, that the particular control configurations discussed herein are provided merely by way of example.

In some embodiments, the burner section 104 may include at least one burner assembly 126, which may have one or more individual components therein (e.g., a burner, an air plenum, air registers, valves, conduits, manifolds, and other components as will be understood by those skilled in the art). In some embodiments, the at least one burner assembly 126 may be connected proximate the base 106 and within the heater shell 102 to combust the fuel when supplied thereto, thereby generating the flue gas that transfers heat to the process fluid contained within the one or more heating coils 114. While the embodiment depicted in FIG. 1 only includes a single burner assembly 126 for the burner section 104, it should be noted that multiple burner assemblies may be included in the burner section 104 of various embodiments of the disclosure. For example, in certain embodiments, the natural draft heater may comprise a plurality of burner assemblies having various configurations and/or arrangements within the burner section of the heater shell. In some embodiments, the natural draft heater may comprise at least one burner assembly, at least 2 burner assemblies, at least 3 burner assemblies, at least 4 burner assemblies, at least 5 burner assemblies, at least 6 burner assemblies, at least 7 burner assemblies, at least 8 burner assemblies, or more burner assemblies in the burner section 104 as will be understood for specific applications pursuant to the disclosure.

FIG. 2, for example, depicts a perspective view of a natural draft heater 200 that is configured to have eight separate burner assemblies installed in the burner section 202. Various configurations and arrangements of natural draft heaters, including the positioning of various sections and/or components therein, may vary as will be understood by a person skilled in the art. FIG. 2 illustrates a three-dimensional view of a large scale natural draft heater system according to one or more embodiments of the disclosure. For example, the natural draft heater depicted in FIG. 2 includes a burner section 202, a radiant section 204, a convective section 206, and a stack section 208, which may optionally include a stack damper 210.

In some embodiments, one or more embodiments of natural draft heater systems according to the disclosure (e.g., as depicted in FIGS. 1 and 2) may include at least one burner assembly 212, which may include one or more individual components forming the overall burner assembly (e.g., as depicted in FIGS. 3A, 3B, 4A, 4B, 5A, and 5B). For example, FIG. 3A and FIG. 4A illustrate a side view and a front view, respectively, of a burner assembly for use in a natural draft heater according to the present disclosure. Referring now to FIGS. 3A and 4A, in one or more embodiments of the disclosure, the at least one burner 212 assembly may include a burner 214 positioned within the at least one burner assembly to ignite the fuel when being supplied to the burner. In some embodiments, the at least one burner assembly may include a burner air sensor 128 positioned adjacent the burner to measure a level of excess air, for example, referring back to FIG. 1. In some embodiments, the burner air sensor 128 may be in electrical communication with at least a control component 130 and one or more other components (e.g., a burner air register 218 as shown in FIGS. 3A and 4A) within the at least one burner assembly as will be discussed further herein.

Referring back to FIGS. 3A and 4A, the at least one burner assembly may include an air plenum 216 positioned adjacent the burner to distribute air into the burner. In some embodiments that air plenum may include an air input, i.e., an opening in the air plenum (not pictured), to receive atmospheric air. Generally, the air plenum may work in connection with one or more other components within the at least one burner assembly to deliver air flow directly to the burner. For example, in one or more embodiments, the at least one burner assembly may include a burner air register 218 in fluid communication with the air input to direct the atmospheric air into the air plenum 216. Air registers may be used in burner assemblies for industrial fired heaters, for example, to control the amount of air flow into the system. The methods described herein advantageously include incorporating burner air registers 218 with linear percentage openings to control excess air level in the burner 214 between about 15% to about 25% by weight (based on the combined weight of air and fuel needed for complete combustion) from full design capacity (e.g., heat absorption duty of about 100%) to turn down operating condition range (e.g., heat absorption duty of about 20%). Using a burner air register with linear percentage openings may also provide more accurate control of the excess air level in the burner and/or ease of operation by an operator of natural draft heater.

In some embodiments, for example, the at least one burner assembly may include a burner air register 218 having a housing 218 a, one or more plates 218 b movably attached to the housing and positioned in fluid communication with the air plenum 216 to direct air flow through the air plenum, and a handle 218 c attached to the housing 218 a to adjust the position of the one or more plates 218 b to effectuate a movement thereof. In some embodiments, each of the one or more plates may be configurable between one or more of an open position, a partially open position, and a closed position to selectively supply air to the plenum. In some embodiments, for example, the burner air register may include a plate, or more than one plate, positioned at least partially inside the air plenum and connected to the housing of the burner air register. In such embodiments, air enters the air plenum and flows under or around the one or more plates. Generally, the position of the handle, which may be controlled manually by an operator, may control the level of the plates inside the air plenum. Controlling the position of the one or more plates inside or adjacent the air plenum controls the percentage opening (e.g., on a linear scale) available for air to pass through the air plenum and to the burner. Using this linear scale provides the operator with a visual scale so that the operator may see the percentage opening and adjust this percentage opening based one or more readings from a controller to achieve the desired amount of air flow into the air plenum.

A burner air register as described herein may be referred to in some embodiments as a burner air register with linear percentage opening, for example. Advantageously, use of a burner air register with linear percentage openings may provide more efficient control of the excess air in the burner. Generally, the percentage of opening (which may range from 0%/closed to 100%/completely open) may be increased to allow more ambient or atmospheric air to be introduced into the air plenum and/or the burner; the percentage of opening may be decreased to reduce the amount of ambient air introduced into the air plenum and/or the burner. For example, the one or more plates may be configurable to be about 0% open (e.g., in a closed positioned), at least about 10% open, at least about 20% open, at least about 30% open, at least about 40% open, at least about 50% open, at least about 60% open, at least about 70% open, at least about 80% open, at least about 90% open, or about 100% open (e.g., an open position). In some embodiments, proper use of the burner air register may prevent excess air from being delivered directly into the burner in an uncontrolled manner.

In some embodiments, the at least one burner assembly may include a fuel supply system for delivering the fuel to the burner 214, i.e., the individual burner tips of the burner assembly. For example, the fuel supply system may include a fuel input conduit 234, a primary manifold assembly 232, and a second, staged manifold assembly 222. In some embodiments, the fuel input conduit 234 is positioned in fluid communication with the primary manifold assembly 232 to deliver fuel to the primary manifold in a central location in primary manifold. In some embodiments, the primary manifold 232 is in fluid communication with one or both of a burner tip of a burner 214 and the second, staged manifold assembly 222 to deliver fuel to one or both of the burner tip of the burner 214 and the second, staged manifold assembly 222. In some embodiments, the second, staged manifold assembly 222 may be in fluid communication with another burner tip, i.e., one of the burner tips not connected to the primary manifold, to deliver fuel to the another burner tip. In some embodiments, the second, staged manifold assembly may include a staged manifold valve 222 a configurable to be in a closed position to shut off fuel flow or in an at least partially open position (e.g., between about 1% to about 100% open) to direct a preselected amount of fuel to the another burner tip, i.e., one of the burner tips not connected to the primary manifold, to achieve a desired concentration of air and fuel in the another burner tip. In some embodiments, the fuel supply system may include one or more conduits 236 (e.g., in the form of a conduit, tube, pipe, etc.) in fluid communication with one or more of the primary manifold assembly 232, the second, staged manifold assembly 222, and each of the burner tips of the burner 214 to deliver fuel to one or more of those particular components within the at least one burner assembly.

In some embodiments, the at least one burner assembly includes at least one burner fuel tip 226 positioned within each burner 214 and a riser plate 238 positioned adjacent thereto at least partially enclosing the burner fuel tip. In some embodiments, the riser plate 238 may have a burner riser 242 positioned proximate a base thereof and connected thereto by one or more riser welds 244 to allow air to enter the riser plate 238 between riser welds during operation of the natural draft heater. In some embodiments, the at least one burner assembly may include a plurality of burner fuel tips positioned within the burner assembly 212, for example, as depicted in FIGS. 4A and 5A. In such embodiments, each of the plurality of burner fuel tips may be at least partially enclosed by a separate riser plate and have a separate burner riser positioned proximate the base thereof and connected thereto by one or more riser welds. Generally, the number of burner fuel tips in the burner assembly may vary. For example, the burner assembly may include at least 1 burner fuel tips, at least 2 burner fuel tips, at least 3 burner fuel tips, at least 4 burner fuel tips, or more as would be understood by a skilled person in the art.

Referring now to FIGS. 5A and 5B (e.g., showing an exploded view of an air ring according to the disclosure), in one or more embodiments, the at least one burner assembly may include one or more air rings 220 positioned about the burner riser 242 proximate the one or more riser welds 244 to reduce air entering the riser plate. Typically, air flow into the burner (e.g., via gaps between riser welds 244 on the riser plate 238) may affect the stoichiometry of the air and fuel locally at the burner. Generally, the air rings 220 are designed to reduce the size of the gap that lets air flow up to burner tip 226 located at burner 214. In some embodiments, the air rings are constructed of metal (e.g., such as stainless steel) and are designed to withstand high heat exposure during use. Advantageously, the air rings (when installed at the upper operating rates (e.g., heat absorption duty greater than about 40%) versus not installed at the lower turn down rates (e.g. heat absorption duty less than about 40%) may beneficially contribute to the ability to control NO_(x) as offset with CO formation. However, the number of air rings and the positioning thereof may generally vary based on the configuration of the natural draft heater.

In some embodiments, installation of the air rings may provide a further range of adjustment of the air flow to the individual burner tips, beyond that which the air register inflow provides. Upon installation of the air rings, the small opening between riser welds, typically allowing ambient air to enter the burner tip, is substantially blocked by the air ring, thereby reducing the amount of oxygen available for combustion at each individual burner tip within the burner. In some embodiments, the air rings may be designed to be removed, for example, removal of the air rings may be necessary under planned start up and/or shut down of one or more burner tips and/or the entire burner assembly. For example, in some embodiments, the one or more air rings may be selectively installed in one of the at least one burner assemblies when that one burner assembly is taken out of service during operation of the natural draft heater so as to prevent air leakage into the burner and/or the heater shell and thereby reduce NO_(x) formation. In such embodiments, the air rings may be configured to be separately removed from individual burner assemblies. In some embodiments, the air rings may be installed and uninstalled during operation of the natural draft heater, which may provide for some degree of emissions control during use of the heater. For example, uninstalling the air rings during operation may provide a reduction in CO production locally at the burner; however, this reduction comes at the cost of NO_(x) emissions at the main heater stack outlet.

In some embodiments, a silicone seal (not pictured) may be applied to at one or more connections between the burner risers 242, the riser plates 238, and the riser welds 244 (e.g., as depicted by the circled portion in FIG. 5A). Generally, as noted above, the connections between the burner risers, the riser plates, and the riser are not air tight and may have gaps therein. For example, these connection points in common burner assemblies may undesirably allow air penetration into the burner and/or the heater shell which has an undesirable effect on NO_(x) and CO emissions therefrom. However, application of high temperature silicone seals to one or more of these connection points within the at least one burner assembly may advantageously prevent unwanted air infiltration into the heater thereby reducing NO_(x) formation in the heater shell. Application of the high temperature silicone seals may vary in application methods and/or configuration, for example, the high temperature silicone seal may be in the form of a silicone gasket and/or a silicone sleeve and/or a silicone seal and may be applied via any method commonly used in the art as would be known by a person skilled in the art.

In some embodiments, the at least one burner assembly 212 may include a flame scanner 224 positioned adjacent the air plenum 216 to detect the presence of a flame in the burner 214, for example, as depicted in FIGS. 3A, 4A, and 4B (e.g., showing a cut away view of the flame scanner 224 attached to the air plenum 216). Generally, a flame scanner operates as an electric eye looking at the flame to ensure the burner is producing a flame therefrom. For example, the air plenum 216 has a generally hollow interior allowing the flame scanner 224 to observe a flame in the burner from a distance below the actual burner. Any type of flame scanner typically used in natural draft heaters as would be understood by a person of skill in the art may be suitable for use in the natural draft heaters described herein.

In some embodiments, the flame scanner may also be configured to allow a small amount of purge air into the flame scanner, e.g., such that the purge air blows across the lens of the flame scanner to keep the lens of the flame scanner clear of dust that may fall from within the heater shell. For example, in the embodiment depicted in FIG. 6, the at least one burner assembly 212 includes a purge air injection input 246 positioned adjacent the flame scanner 224 to deliver purge air directly to the burner. Such a configuration advantageously allows the purge air to be used as combustion air by the burner thereby reducing NO_(x) formation within the heater shell. For example, since the air being injected into the flame scanner ultimately passes to the burner within the burner assembly, it combines with the combustible air in the burner assembly. Such a configuration is not typically used in industry as flame scanners are commonly known in the art to be used in other locations, e.g., such as the side walls of the heater shell. In some embodiments, the flame scanners can be mounted at every other burner assembly (e.g. in a natural draft heater having three or more burner assemblies) which ensures that a flame is maintained the plurality of burner assemblies. Without intending to be bound by theory, it should be noted that, if the flame scanners were mounted on the side walls of the furnace as is typically done, then the air would not be combustible air. In such a scenario, the purge air undesirably contributes to higher NO_(x) emissions exiting the stack.

As noted above, in some embodiments, the burner section of the natural draft heater may include a plurality of burner assemblies. In some embodiments, the flame scanner and the purge air injection input may be included on alternating burner assemblies when the at least one burner assembly includes at least two or more burner assemblies. For example, in some embodiments, the at least one burner assembly may include eight burner assemblies and every other burner assembly (four in total) may include a flame scanner mounted within the burner assembly looking upwards at the burner tips to ensure that the burner tips are operating correctly and a purge air injection input adjacent thereto. In some embodiments, for example, the flame scanner and/or the purge air injection input may be absent on at least one burner assembly when the at least one burner assembly includes two or more burner assemblies. In other embodiments, the flame scanner and/or the urge air injection input may be located at every burner assembly.

In some embodiments, the at least one burner assembly may include a pilot burner configuration therein to ensure that the burner remains firing. Referring back to FIGS. 3A and 3B, for example, the at least one burner assembly 212 may include a pilot burner tip 228 in fluid communication with a pilot gas connection 230. In some embodiments, the pilot burner tip 228 may be positioned within the burner 214 to provide a continuous pilot flame within the burner during operation of the natural draft heater. In some embodiments, the pilot gas connection 230 may provide a continuous flow of gas to the pilot burner 230 during operation of the natural draft heater.

As shown in FIG. 1 and as noted above, in one or more embodiments, a natural draft heater system as described herein may include a controller 130 in electrical communication with one or more component within the natural draft heater. For example, the controller may be in electrical communication with the draft sensor 116 and/or the burner air sensor 128. In some embodiments, the controller may also be in communication with the burner air register 218 (e.g., referring to FIG. 3) and/or the one or more actuators 306 (e.g., referring to FIG. 7) in the split range stack damper. In some embodiments, the controller may be in electrical communication with one or more components of the natural draft heater. In some embodiments, the controller 130 may receive an input signal representative of negative pressure of the flue gas from the draft sensor 116 and provide an output signal (from the controller) to the one or more actuators 306 in the split-range stack damper to adjust the positioning of the set of inner blades 302 and the set of outer blades 304 of the split-range stack damper and thereby maintain the bridge wall draft value (e.g., negative pressure within the heater shell proximate the bridge wall) within a preselected range. In some embodiments, the preselected range of the bridge wall draft value may be maintained in the range of about 0.5-inches water-column to about 0.15-inches water-column, or about 0.10-inches water-column to about 0.15-inches water-column to provide the desired negative pressure within the natural draft heater. In some embodiments, the selected preselected range of the bridge wall draft value may be maintained at about 0.10-inches water-column to provide the desired negative pressure within the natural draft heater. In some embodiments, the controller may receive an input signal representative of excess air level from the burner air sensor 128 and provide an alert to an operator to adjust configuration of the one or more plates 218 b and thereby control excess air in the burner 214 to a value in the range of between about 15% to about 25% by weight, based on the combined weight of air and fuel needed for complete combustion.

As noted above, some aspects of the disclosure relate to methods of reducing NO_(x) and CO emissions in a natural draft heater. In one or more embodiments, methods of reducing NO_(x) and CO emissions in a natural draft heater may include measuring a selected draft value related to the negative pressure of flue gas within the heater shell when operation of the natural draft heater occurs. In some embodiments, such methods may include adjusting a split-range stack damper associated with the natural draft heater to maintain the selected draft value to within a preselected range. For example, in some embodiments according to the disclosure, the preselected range of the selected draft value is maintained in the range of about 0.5-inches water-column to about 0.15-inches water-column, or about 0.10-inches water-column to about 0.15-inches water-column to provide the desired negative pressure within the natural draft heater. In some embodiments, the selected draft value may be maintained at about 0.10-inches water-column to provide the desired negative pressure within the natural draft heater. Generally, a split-range stack damper as described herein above with respect to the natural draft heater systems of this disclosure may also be suitable for use in one or more methods as described herein. For example, in some embodiments, the split-range stack damper may include a set of inner blades, a set of outer blades, and one or more actuators arranged to actuate the set of inner blades and the set of outer blades to thereby effectuate movement thereof to maintain the selected draft value within the preselected range.

In some embodiments, one or more methods as described herein may include measuring an excess air level in a burner when operation of the natural draft heater occurs. In some embodiments, such methods may include controlling excess air in a burner associated with the natural gas heater to a value in the range of between about 15% to about 25% by weight based on the combined weight of air and fuel needed for complete combustion within the burner. In some embodiments, such methods may include controlling the draft within the heater shell within a preselected range thereby to control the amount of excess air in the burner and maintain at least one of NO_(x) emissions not exceeding 0.025 lb/MMBtu (HHV) and CO emissions not exceeding 0.01 lb/MMBtu (HHV) in the natural draft heater.

In some embodiments, one or methods as described herein may include detecting the presence of a flame in the burner when operation of the natural draft heater occurs.

Generally, a flame scanner as described herein above with respect to the natural draft heater systems of this disclosure may also be suitable for use in one or more methods as described herein. For example, in some embodiments, the flame scanner may be positioned directly in the burner. In some embodiments, such methods may include injecting a purge air directly into the burner, the purge air being used as combustion air by the burner to thereby reduce NO_(x) formation within the heater shell.

In some embodiments, one or more methods as described herein may include sealing one or more connections proximate the burner to prevent air infiltration into the burner to thereby reduce NO_(x) formation in the heater shell. In some embodiments, the one or more connections may be sealed using air rings and/or a high temperature silicone seal, for example, as referred to herein above with respect to the natural draft heater systems described herein.

In some embodiments, one or more methods described herein include selecting adequate burner and tube circle diameters to induce proper flue gas circulation in the furnace. Generally, the burner and tube circle diameters may vary in commercial natural draft heaters based on fabrication specifications and other design parameters and these specifications may not provide adequate circulation of combustion gases within the burner assembly. Advantageously, the burner and tube diameters may be specifically selected for new fabrications, specifically to ensure an adequate amount of flue gas is internally recirculated and mixed with the combustion air to the burner to provide the desired combustion level within the heater, for example, to ensure complete combustion. The internal recirculation of the flue gas reduces the flame temperature, consequently, decreasing the thermal NO_(x) emissions in the system.

Finally, one or more methods described herein may also include selectively turning burners out of service at low firing rates to enhance local mixing of air and fuel for in-service burners. Typically, natural draft heaters are operated under a wide range of conditions, for example, based on the desired output from the overall heater assembly. Thus, in times when the heaters are firing at full capacity, or at some degree of reduced capacity, the NO_(x) and CO emissions may vary greatly. The methods provided herein allow for an operator to selectively turn one or more burner assemblies completely out of service (as opposed to just running at a lower capacity) when the overall heater firing rate is low so as to provide enhanced local mixing of air and fuel in the burner assemblies that remain in service (e.g., because these burner assemblies can run at a higher capacity).

Any one of the methods according to the present disclosure may optionally include providing a natural draft heater having various components and/or design modifications and/or operation schemes as noted herein above with respect to the natural draft heater systems of the present disclosure. Such methods may include providing a natural draft heater including various components noted herein above, including, but not limited to, a heater shell, a bridge wall, one or more heating coils, a draft sensor, a stack (e.g., including a split-range stack damper), at least one burner assembly (e.g., including one or more of a burner, a burner air sensor, an air plenum, a burner air register, and a fuel supply system), and a controller.

In one or more embodiments a method, for example, may include providing a natural draft heater including a heater shell designed to circulate a flue gas internally therein via negative pressure, the flue gas being generated by combustion of a fuel within the heater shell. In some embodiments, for example, the heater shell may include a base and a burner section positioned proximate the base where combustion of the fuel occurs. In some embodiments, the heater shell may include a radiant section positioned adjacent the burner section to receive heat energy from the burner section and radiate heat energy therefrom and a convective section positioned adjacent the radiant section to provide convection from the radiant section.

In some embodiments, the natural draft heater provided according to the methods provided herein may include a bridge wall connected to the heater shell and positioned between the radiant section and the convective section thereof. In some embodiments, the natural draft heater provided according to the methods provided herein may include one or more heating coils positioned within the heater shell proximate one or both of the radiant section and the convective section. For example, the one or more heating coils may contain a process fluid and the one or more heating coils may be arranged to transfer heat from the circulated flue gas to thereby heat the process fluid. In some embodiments, the natural draft heater provided according to the methods provided herein may include a draft sensor positioned proximate the bridge wall to measure a bridge wall draft value related to the negative pressure of the flue gas within the heater shell.

In some embodiments, the natural draft heater provided according to the methods provided herein may include a stack positioned proximate the convective section of the heater shell for venting of at least a portion of the circulated flue gas to atmosphere. In some embodiments, the stack may include an outer shell and a split-range stack damper positioned within the outer shell to maintain the bridge wall draft of the flue gas being vented from the natural draft heater. In some embodiments, the split-range stack damper may include a set of inner blades, a set of outer blades, and one or more actuators arranged to actuate the set of inner blades and the set of outer blades to thereby effectuate movement thereof.

In some embodiments, the natural draft heater provided according to the methods provided herein may include at least one burner assembly connected proximate the base of the heater shell to combust the fuel when supplied thereto, thereby generating the flue gas that transfers heat to the process fluid contained within the one or more heating coils. In some embodiments, the at least one burner assembly may include one or more components therein. For example, the at least one burner assembly may have a burner positioned within the at least one burner assembly to ignite the fuel when being supplied to the burner. In some embodiments, the at least one burner assembly may include a burner air sensor positioned adjacent the burner to measure a level of excess air. In some embodiments, the at least one burner assembly may include an air plenum adjacent to the burner to distribute air into the burner, the air plenum including an air input to receive atmospheric air. In certain embodiments, the at least one burner assembly may include a burner air register in fluid communication with the air input to direct the atmospheric air into the air plenum. For example, in some embodiments, the burner air register may have a housing, one or more plates attached to the housing and positioned in fluid communication with the air plenum to direct air flow into the air plenum, and a handle attached to the housing to adjust the position of the one or more plates to effectuate a movement thereof. In certain other embodiments, each of the one or more plates may be configurable between one or more of an open position, a partially open position, and a closed positioned to selectively supply air to the air plenum.

In some embodiments, the at least one burner assembly may include a fuel supply system. For example, the fuel supply system may have a fuel input conduit, a primary manifold assembly, and a second, staged manifold assembly. In some embodiments, the first input conduit may be positioned in fluid communication with the primary manifold assembly to deliver fuel to the primary manifold assembly. In some embodiments, the primary manifold assembly may be positioned in fluid communication with one or both of a burner tip of the burner and the second, staged manifold assembly to deliver fuel to one or both of the burner tip of the burner and the second, staged manifold. In some embodiments, the second, staged manifold assembly may be positioned in fluid communication with another burner tip to deliver fuel to the another burner tip. In certain embodiments, the second, staged manifold assembly may include a staged manifold valve that may be configurable to be in a closed position to shut off fuel flow or in an at least partially open position to direct a preselected amount of fuel to the another burner tip to achieve a desired concentration of air and fuel mixture in the another burner tip.

In one or more embodiments a method, for example, may include providing a natural draft heater including a controller in electrical communication with one or more components within the natural draft heater. For example, in some embodiments, the controller may be in electrical communication with the burner air register, the burner air sensor, the draft sensor, and the one or more actuators in the split-range stack damper.

As noted herein, the methods and systems according to the present disclosure may provide a reduction in one or both of the NO_(x) and CO emissions from a natural draft heater. CO and NO_(x) emissions are typically known to provide a trade-off, for example, reducing CO emissions may result in a subsequent increase in NO_(x) emissions and vis-a-versa. Therefore, selection of the desired process modification according to the methods disclosed herein may depend on the desired NO_(x) and/or CO emissions to be achieved, Advantageously, it has been discovered that a natural draft heater including all of the additional process steps as described herein above may demonstrate reduction of both NO_(x) and CO emissions simultaneously to a very high degree, for example, exhibiting NO_(x) emissions not exceeding 0.025 lb/MMBtu (HHV) and CO emissions not exceeding 0.01 lb/MMBtu (HHV). The table provided in FIG. 8 shows predicted emissions guarantees for natural draft heater systems according to the disclosure including the design features and operational schemes provided herein above, indicating that the predicted NO_(x) emissions will not exceed 0.025 lb/MMBtu (HHV) and the predicted CO emissions will not exceed 0.01 lb/MMBtu (HHV). As depicted in FIG. 8, guarantees are provided for operation of the natural draft heater system under design capacity with 15% excess air (“Design w/15% Excess Air), design capacity with 25% excess air (“Design w/25% Excess Air), normal start of run conditions (“Normal SOR”), normal end of run conditions (“Normal EOR”), and end of run turndown condition (“EOR-Turndown”).

EXPERIMENTAL

Prototype testing was conducted based on a single burner assembly configuration in a test furnace simulating the thermal profile of a natural draft heater. Testing was conducted in the test furnace to determine the impact of various design parameters on NO_(x) emissions using two different types of fuels, for example, using a liquefied petroleum gas (LPG) fuel gas and a low BTU fuel gas. The particular design parameters evaluated included the effect of air rings (installed or uninstalled), the effect of closing the staged manifold valves, and the impact on excess air. It should be noted that the results presented herein are not intended to be limiting of embodiments of the systems and methods of the present disclosure as will be understood by those skilled in the art, and the particular results presented herein are presented by way of example alone. Generally, it should be noted that actual magnitudes of impact on a natural draft heater by various design methods may be varied based on the actual heater furnace geometry and/or the number of burners in operation and/or the particular configuration of the heater itself.

FIG. 9 illustrates a graph showing the impact of various burner design parameters on NOR emissions using a LPG fuel gas during prototype testing. As illustrated in FIG. 9, the results of prototype testing show that removing the air rings during operation, closing the staged fuel valves during operation, and operating the heaters at high excess air levels (e.g., in the range of about 15% to about 25% excess air by weight, based on the combined weight of the air and fuel to be combusted) generally increased NO_(x) emissions at higher operating conditions within the heater. However, this increase in NO_(x) emissions resulted in reduction of CO emissions during testing. It should also be noted that the NO_(x) emissions appeared to decrease at higher firing rates when the excess air level in the heater was maintained at about 25% excess air. In addition, as demonstrated in FIG. 9, when air rings were installed during prototype testing the NO_(x) emissions decreased when the excess air level in the heater was maintained at 25% excess air.

FIG. 10 illustrates a graph showing the impact of various burner design parameters on NO_(x) emissions using a low BTU fuel gas during prototype testing. As illustrated in FIG. 9, the results of prototype testing show that removing the air rings during operation, closing the staged fuel valves during operation, and operating the heaters at high excess air levels (e.g., in the range of about 15% to about 25% excess air by weight, based on the combined weight of the air and fuel to be combusted) generally increased NO_(x) emissions at higher operating conditions within the heater. However, this increase in NO_(x) emissions resulted in reduction of CO emissions during testing. It should also be noted that the NO_(x) emissions appeared to decrease at higher firing rates when the excess air level in the heater was maintained at about 25% excess air.

All NO_(x) emissions values presented in FIGS. 9 and 10 are represented in units of parts per million (ppm) NO_(x), dry and corrected to 3% O2. All heater firing rates/operation capacities presented in FIGS. 9 and 10 are represented in units of MMBtu/hour lower heating value (LHV).

Having the benefit of the teachings presented in the foregoing descriptions, many modifications and other embodiments of the disclosure set forth herein will come to mind to those skilled in the art to which these disclosures pertain. Therefore, it is to be understood that the disclosure is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation. 

1. A natural draft heater system comprising: a heater shell having a base and designed to circulate a flue gas internally therein via negative pressure, the flue gas being generated by combustion of a fuel within the heater shell; one or more heating coils positioned within the heater shell, the one or more heating coils containing a process fluid and being arranged to transfer heat from the circulated flue gas to thereby heat the process fluid; a draft sensor positioned within the heater shell to measure a negative pressure of the flue gas within the heater shell during operation of the natural draft heater; a stack attached to the heater shell for venting of at least a portion of the circulated flue gas to atmosphere, the stack including an outer shell and a split-range stack damper positioned within the outer shell to maintain a negative pressure of the flue gas being vented from the natural draft heater, the split-range stack damper including a set of inner blades, a set of outer blades, and one or more actuators arranged to actuate the set of inner blades and the set of outer blades to thereby effectuate movement thereof; at least one burner assembly connected proximate the base of and within the heater shell to combust the fuel when supplied thereto, thereby generating the flue gas that transfers heat to the process fluid contained within the one or more heating coils, the at least one burner assembly having: a burner positioned within the at least one burner assembly to ignite the fuel when being supplied to the burner, a burner air sensor positioned adjacent the burner to measure a level of excess air, an air plenum adjacent to the burner to distribute air into the burner, the air plenum including an air input to receive atmospheric air, a burner air register in fluid communication with the air input to direct the atmospheric air into the air plenum, the burner air register having a housing, one or more plates attached to the housing and positioned in fluid communication with the air plenum to direct air flow into the plenum, and a handle attached to the housing to adjust the position of the one or more plates to effectuate a movement thereof, each of the one or more plates being configurable between one or more of an open position, a partially open position, and a closed position to selectively supply air to the air plenum, and a fuel supply system including at least a fuel input conduit to deliver fuel to the burner, and a controller in electrical communication with the burner air register, the burner air sensor, the draft sensor, and the one or more actuators in the split-range stack damper to control the negative pressure of the flue gas within the heater shell to deliver an amount of excess air to the burner to thereby maintain at least one of NO_(x) emissions not exceeding 0.025 lb/MMBtu (HHV) and CO emissions not exceeding 0.01 lb/MMBtu (HHV) in the natural draft heater.
 2. The natural draft heater system of claim 1, wherein the controller receives an input signal representative of negative pressure of the flue gas from the draft sensor.
 3. The natural draft heater system of claim 2, wherein the controller provides an output signal to the one or more actuators in the split-range stack damper to adjust the positioning of the set of inner blades and the set of outer blades of the split-range stack damper and thereby maintain the negative pressure of the flue gas to within a preselected range.
 4. The natural draft heater system of claim 3, wherein the preselected range is maintained in the range of about 0.10-inches water-column to about 0.15-inches water-column.
 5. The natural draft heater system of claim 3, wherein the output signal is in the form of a current delivered to the one or more actuators, the current being in the range of about 2 mA to about 20 mA, the one or more actuators being arranged to adjust the position of at least one of the set of inner blades and the set of outer blades in response to the current received.
 6. The natural draft heater system of claim 1, wherein the controller receives an input signal representative of excess air level from the burner air sensor.
 7. The natural draft heater system of claim 6, wherein the controller alerts an operator to adjust configuration of the one or more plates and thereby control the excess air in the burner to a value in the range of between about 15% to about 25% by weight based on the combined weight of air and fuel needed for complete combustion.
 8. The natural draft heater system of claim 1, wherein the heater shell includes a burner section positioned proximate the base where combustion of the fuel occurs, a radiant section positioned adjacent the burner section to receive heat energy from the burner section and radiate heat energy therefrom, and a convective section positioned adjacent the radiant section to provide convection from the radiant section.
 9. The natural draft heater system of claim 8, further comprising a bridge wall connected to the heater shell and positioned between the radiant section and the convective section thereof.
 10. The natural draft heater system of claim 9, wherein the draft sensor is positioned proximate the bridge wall such that the negative pressure of the flue gas is measured proximate the location of the bridge wall.
 11. The natural draft heater system of claim 1, wherein the fuel supply system further comprises a primary manifold assembly and a second, staged manifold assembly, the fuel input conduit positioned in fluid communication with the primary manifold assembly to deliver fuel to the primary manifold assembly, the primary manifold assembly positioned in fluid communication with one or both of a burner tip of the burner and the second, staged manifold assembly to deliver fuel to one or both of the burner tip of the burner and the second, staged manifold, the second, staged manifold assembly positioned in fluid communication with another burner tip to deliver fuel to the another burner tip.
 12. The natural draft heater system of claim 11, wherein the second, staged manifold assembly includes a staged manifold valve configurable to be in a closed position to shut off fuel flow or in an at least partially open position to direct a preselected amount of fuel to the another burner tip to achieve a desired concentration of air and fuel mixture in the another burner tip.
 13. The natural draft heater system of claim 1, wherein the at least one burner assembly is a plurality of burner assemblies positioned in sequence along the base of and within the heater shell to provide substantially even heating within the heater shell, the plurality of burner assemblies are independently controlled and operated during operation of the natural draft heater, such that one or more of the plurality of burner assemblies may be taken out of service during operation of the natural draft heater while the remaining burner assemblies remain operational.
 14. The natural draft heater system of claim 1, further comprising a burner fuel tip positioned within the burner and at least partially enclosed by a riser plate positioned adjacent thereto, the riser plate having a burner riser positioned proximate a base thereof and connected thereto by one or more riser welds to allow air to enter the riser plate between riser welds during operation of the natural draft heater.
 15. The natural draft heater system of claim 14, further comprising one or more air rings positioned about the burner riser proximate the one or more riser welds to reduce air entering the riser plate.
 16. The natural draft heater system of claim 15, wherein the one or more air rings are positioned to reduce air entering the riser plate when the natural draft heater is operating at rates greater than about 40% of design capacity.
 17. The natural draft heater system of claim 15, wherein the one or more air rings are selectively installed in one of the at least one burner assembly when that one burner assembly is taken out of service during operation of the natural draft heater so as to prevent air leakage into the heater shell and thereby reduce NO_(x) formation in the heater shell.
 18. The natural draft heater system of claim 14, further comprising a silicone seal applied at the connection between the burner risers and the riser plates to prevent air infiltration into the burner thereby reducing NO_(x) formation in the heater shell.
 19. The natural draft heater system of claim 1, further comprising a flame scanner positioned adjacent the air plenum to detect the presence of a flame in the burner.
 20. The natural draft heater system of claim 19, further comprising a purge air injection input positioned adjacent the flame scanner to deliver purge air directly to the burner, the purge air being used as combustion air by the burner thereby reducing NO_(x) formation within the heater shell.
 21. The natural draft heater system of claim 20, wherein the flame scanner and the purge air injection input are included on alternating burner assemblies when the at least one burner assembly includes two or more burner assemblies.
 22. The natural draft heater system of claim 20, wherein the flame scanner and the purge air injection input are absent on at least one burner assembly when the at least one burner assembly includes two or more burner assemblies.
 23. A natural draft heater system comprising: a heater shell designed to circulate a flue gas internally therein via negative pressure, the flue gas being generated by combustion of a fuel within the heater shell, the heater shell including a base, a burner section positioned proximate the base where combustion of the fuel occurs, a radiant section positioned adjacent the burner section to receive heat energy from the burner section and radiate heat energy therefrom, and a convective section positioned adjacent the radiant section to provide convection from the radiant section, a bridge wall connected to the heater shell and positioned between the radiant section and the convective section, one or more heating coils positioned within the heater shell proximate one or both of the radiant section and the convective section, the one or more heating coils containing a process fluid and being arranged to transfer heat from the circulated flue gas to thereby heat the process fluid; a draft sensor positioned proximate the bridge wall to measure a bridge wall draft value related to the negative pressure of the flue gas within the heater shell during operation of the natural draft heater; a stack positioned proximate the convective section of the heater shell for venting of at least a portion of the circulated flue gas to atmosphere, the stack including an outer shell and a split-range stack damper positioned within the outer shell to maintain the bridge wall draft of the flue gas being vented from the natural draft heater, the split-range stack damper including a set of inner blades, a set of outer blades, and one or more actuators arranged to actuate the set of inner blades and the set of outer blades to thereby effectuate movement thereof; at least one burner assembly connected proximate the base of and within the heater shell to combust the fuel when supplied thereto, thereby generating the flue gas that transfers heat to the process fluid contained within the one or more heating coils, the at least one burner assembly having: a burner positioned within the at least one burner assembly to ignite the fuel when being supplied to the burner, a burner air sensor positioned adjacent the burner to measure a level of excess air, an air plenum adjacent to the burner to distribute air into the burner, the air plenum including an air input to receive atmospheric air, a burner air register in fluid communication with the air input to direct the atmospheric air into the air plenum, the burner air register having a housing, one or more plates attached to the housing and positioned in fluid communication with the air plenum to direct air flow into the plenum, and a handle attached to the housing to adjust the position of the one or more plates to effectuate a movement thereof, each of the one or more plates being configurable between one or more of an open position, a partially open position, and a closed position to selectively supply air to the air plenum, and a fuel supply system including a fuel input conduit, a primary manifold assembly, and a second, staged manifold assembly, the fuel input conduit positioned in fluid communication with the primary manifold assembly to deliver fuel to the primary manifold assembly, the primary manifold assembly positioned in fluid communication with one or both of a burner tip of the burner and the second, staged manifold assembly to deliver fuel to one or both of the burner tip of the burner and the second, staged manifold, the second, staged manifold assembly positioned in fluid communication with another burner tip to deliver fuel to the another burner tip, the second, staged manifold assembly including a staged manifold valve configurable to be in a closed position to shut off fuel flow or in an at least partially open position to direct a preselected amount of fuel to the another burner tip to achieve a desired concentration of air and fuel mixture in the another burner tip, and a controller in electrical communication with the burner air register, the burner air sensor, the draft sensor, and the one or more actuators in the split-range stack damper to (a) control the bridge wall draft within the heater shell to deliver an amount of excess air to the burner to thereby maintain at least one of NO_(x) emissions not exceeding 0.025 lb/MMBtu (HHV) and CO emissions not exceeding 0.01 lb/MMBtu (HHV) in the natural draft heater, (b) receive an input signal representative of negative pressure of the flue gas from the draft sensor, (c) provide an output signal to the one or more actuators in the split-range stack damper to adjust the positioning of the set of inner blades and the set of outer blades of the split-range stack damper and thereby maintain the bridge wall draft value to within a preselected range, (d) receive an input signal representative of excess air level from the burner air sensor, and (e) provide an alert to an operator to adjust configuration of the one or more plates and thereby control the excess air in the burner to a value in the range of between about 15% to about 25% by weight based on the combined weight of air and fuel needed for complete combustion.
 24. The natural draft heater system of claim 23, wherein the preselected range of the bridge wall draft value is maintained in the range of about 0.10-inches water-column to about 0.15-inches water-column to provide the desired negative pressure within the natural draft heater during operation.
 25. The natural draft heater system of claim 23, wherein the at least one burner assembly is a plurality of burner assemblies positioned in sequence along the base of and within the heater shell to provide substantially even heating within the heater shell, the plurality of burner assemblies are independently controlled and operated during operation of the natural draft heater, such that one or more of the plurality of burner assemblies may be taken out of service during operation of the natural draft heater while the remaining burner assemblies remain operational.
 26. The natural draft heater system of claim 23, further comprising a burner fuel tip positioned within the burner and a riser plate positioned adjacent thereto at least partially enclosing the burner fuel tip, the riser plate having a burner riser positioned proximate a base thereof and connected thereto by one or more riser welds to allow air to enter the riser plate between riser welds during operation of the natural draft heater.
 27. The natural draft heater system of claim 26, further comprising one or more air rings positioned about the burner riser proximate the one or more riser welds to reduce air entering the riser plate.
 28. The natural draft heater system of claim 27, wherein the one or more air rings are positioned to reduce air entering the riser plate when the natural draft heater is operating at rates greater than about 40% of design capacity.
 29. The natural draft heater system of claim 27, wherein the one or more air rings are selectively installed in one of the at least one burner assembly when that one burner assembly is taken out of service during operation of the natural draft heater so as to prevent air leakage into the heater shell and thereby reduce NO_(x) formation in the heater shell.
 30. The natural draft heater system of claim 26, further comprising a silicone seal applied at the connection between the burner risers and the riser plates to prevent air infiltration into the burner thereby reducing NO_(x) formation in the heater shell.
 31. The natural draft heater system of claim 23, further comprising a flame scanner positioned adjacent the air plenum to detect the presence of a flame in the burner.
 32. The natural draft heater system of claim 31, further comprising a purge air injection input positioned adjacent the flame scanner to deliver purge air directly to the burner, the purge air being used as combustion air by the burner thereby reducing NO_(x) formation within the heater shell.
 33. The natural draft heater system of claim 32, wherein the flame scanner and the purge air injection input are included on alternating burner assemblies when the at least one burner assembly includes two or more burner assemblies.
 34. The natural draft heater system of claim 32, wherein the flame scanner and the purge air injection input are absent on at least one burner assembly when the at least one burner assembly includes two or more burner assemblies.
 35. The natural draft heater system of claim 23, wherein the output signal is in the form of a current delivered to the one or more actuators from the controller, the current being in the range of about 2 mA to about 20 mA, the one or more actuators being arranged to adjust the position of at least one of the set of inner blades and the set of outer blades in response to the current received from the controller.
 36. The natural draft heater system of claim 23, wherein the set of inner blades and the set of outer blades in the split-range stack damper are configured to be fully open during startup of the natural draft heater to maximize the bridge wall draft value and the amount of excess air in the burner.
 37. A method of reducing at least one of NO_(x) and CO emissions in a natural draft heater, the method comprising: measuring a selected draft value related to the negative pressure of flue gas within the heater shell when operation of the natural draft heater occurs; adjusting a split-range stack damper associated with the natural draft heater to maintain the selected draft value to within a preselected range; measuring an excess air level in a burner when operation of the natural draft heater occurs; controlling excess air in a burner associated with the natural draft heater to a value in the range of between about 15% to about 25% by weight based on the combined weight of air and fuel needed for complete combustion within the burner; and controlling the draft within the heater shell within a preselected range thereby to control the amount of excess air in the burner and maintain at least one of NO_(x) emissions not exceeding 0.025 lb/MMBtu (HHV) and CO emissions not exceeding 0.01 lb/MMBtu (HHV) in the natural draft heater.
 38. The method of claim 37, wherein the preselected range of the selected draft value is maintained in the range of about 0.10-inches water-column to about 0.15-inches water-column to provide the desired negative pressure within the natural draft heater.
 39. The method of claim 37, wherein the split-range stack damper comprises a set of inner blades, a set of outer blades, and one or more actuators arranged to actuate the set of inner blades and the set of outer blades to thereby effectuate movement thereof to maintain the selected draft value to within the preselected range.
 40. The method of claim 37, further comprising detecting the presence of a flame in the burner when operation of the natural draft heater occurs.
 41. The method of claim 40, further comprising injecting a purge air directly into the burner, the purge air being used as combustion air by the burner to thereby reduce NO_(x) formation within the heater shell.
 42. The method of claim 37, further comprising sealing one or more connections proximate the burner to prevent air infiltration into the burner to thereby reduce NO_(x) formation in the heater shell.
 43. A method of reducing at least one of NO_(x) and CO emissions in a natural draft heater, the method comprising: providing a natural draft heater including: a heater shell designed to circulate a flue gas internally therein via negative pressure, the flue gas being generated by combustion of a fuel within the heater shell, the heater shell including a base, a burner section positioned proximate the base where combustion of the fuel occurs, a radiant section positioned adjacent the burner section to receive heat energy from the burner section and radiate heat energy therefrom, and a convective section positioned adjacent the radiant section to provide convection from the radiant section, a bridge wall connected to the heater shell and positioned between the radiant section and the convective section, one or more heating coils positioned within the heater shell proximate one or both of the radiant section and the convective section, the one or more heating coils containing a process fluid and being arranged to transfer heat from the circulated flue gas to thereby heat the process fluid; a draft sensor positioned proximate the bridge wall to measure a bridge wall draft value related to the negative pressure of the flue gas within the heater shell; a stack positioned proximate the convective section of the heater shell for venting of at least a portion of the circulated flue gas to atmosphere, the stack including an outer shell and a split-range stack damper positioned within the outer shell to maintain the bridge wall draft of the flue gas being vented from the natural draft heater, the split-range stack damper including a set of inner blades, a set of outer blades, and one or more actuators arranged to actuate the set of inner blades and the set of outer blades to thereby effectuate movement thereof; at least one burner assembly connected proximate the base of the heater shell to combust the fuel when supplied thereto, thereby generating the flue gas that transfers heat to the process fluid contained within the one or more heating coils, the at least one burner assembly having: a burner positioned within the at least one burner assembly to ignite the fuel when being supplied to the burner, a burner air sensor positioned adjacent the burner to measure a level of excess air, an air plenum adjacent to the burner to distribute air into the burner, the air plenum including an air input to receive atmospheric air, a burner air register in fluid communication with the air input to direct the atmospheric air into the air plenum, the burner air register having a housing, one or more plates attached to the housing and positioned in fluid communication with the air plenum to direct air flow into the plenum, and a handle attached to the housing to adjust the position of the one or more plates to effectuate a movement thereof, each of the one or more plates being configurable between one or more of an open position, a partially open position, and a closed position to selectively supply air to the air plenum, a fuel supply system including a fuel input conduit, a primary manifold assembly, and a second, staged manifold assembly, the first input conduit positioned in fluid communication with the primary manifold assembly to deliver fuel to the primary manifold assembly, the primary manifold assembly positioned in fluid communication with one or both of a burner tip of the burner and the second, staged manifold assembly to deliver fuel to one or both of the burner tip of the burner and the second, staged manifold, the second, staged manifold assembly positioned in fluid communication with another burner tip to deliver fuel to the another burner tip, the second, staged manifold assembly including a staged manifold valve configurable to be in a closed position to shut off fuel flow or in an at least partially open position to direct a preselected amount of fuel to the another burner tip to achieve a desired concentration of air and fuel mixture in the another burner tip, and a controller in electrical communication with the burner air register, the burner air sensor, the draft sensor, and the one or more actuators in the split-range stack damper; measuring a bridge wall draft value related to the negative pressure of the flue gas within the heater shell via the draft sensor during operation of the natural draft heater; communicating the measured bridge wall draft value to the controller and from the controller to the one or more actuators in the split-range stack damper for adjustment of the position of the set of inner blades and the set of outer blades of the split-range stack damper; adjusting the position of at least one of the set of inner blades and the set of outer blades of the split-range stack damper to maintain the bridge wall draft value to within a preselected range; measuring an excess air level in the burner via a burner air sensor during operation of the natural draft heater; communicating the measured excess air level to the controller whereby the controller alerts an operator to adjust configuration of the one or more plates positioned therein; adjusting the configuration of the one or more plates to control the excess air in the burner to a value in the range of between about 15% to about 25% by weight based on the combined weight of air and fuel needed for complete combustion; controlling the bridge wall draft within the heater shell to deliver an amount of excess air to the burner to thereby maintain at least one of NO_(x) emissions not exceeding 0.025 lb/MMBtu (HHV) and CO emissions not exceeding 0.01 lb/MMBtu (HHV) in the natural draft heater.
 44. The method of claim 43, wherein the preselected range of the bridge wall draft value is maintained in the range of about 0.10-inches water-column to about 0.15-inches water-column to provide the desired negative pressure within the natural draft heater.
 45. The method of claim 43, further comprising detecting the presence of a flame in the burner when operation of the natural draft heater occurs.
 46. The method of claim 45, further comprising injecting a purge air directly into the burner, the purge air being used as combustion air to thereby reduce NO_(x) formation within the heater shell.
 47. The method of claim 46, further comprising detecting the presence of a flame in the burner to thereby inject a purge air into the burner on alternating burner assemblies when the at least one burner assembly includes two or more burner assemblies.
 48. The method of claim 43, further comprising sealing one or more connections within the at least one burner assembly to prevent air infiltration into the burner to thereby reduce NO_(x) formation in the heater shell. 